This invention relates to the production of aqueous suspensions of a natural calcium carbonate.
It is advantageous for some users of fine calcium carbonate materials, especially for paper manufacturers who use such materials as a pigment in paper coating compositions, to receive the material in the form of a deflocculated aqueous suspension since this obviates the need to provide special high power mixing equipment and to consume large quantities of energy. However, if transport of calcium carbonate materials in this manner is to be economically viable the quantity of water mixed with the material should be as small as possible and yet the suspension should be sufficiently fluid to be pumped easily, should not increase in viscosity substantially over a period of several weeks and should not sediment solid particles.
In British Patent Specification No. 1,204,511 there is described a method of producing a suitable aqueous suspension. In this known method calcium carbonate of the required particle size distribution is prepared by a particle size separation technique, for example in a centrifuge, and in the presence of a dispersing, or deflocculating, agent. The deflocculated aqueous suspension of fine calcium carbonate is then flocculated and the flocculated suspension dewatered in, for example, a filter press. The filter cakes are then mixed with a dispersing agent having an organic polymeric anion in a high shear mixer for a time sufficient to dissipate in the resulting slurry at least 5 horsepower hours of energy per ton (13.2. KJKg.sup.-.sup.1) of dry calcium carbonate. This known method provides an aqueous suspension with good properties for transportation but has the disadvantage that dispersing agents of the type described are expensive and a certain minimum quantity of dispersing agent must be added to secure adequate fluidity and stability.
In British Patent Specification No. 1,215,576 there is described a method of producing an aqueous suspension of a calcium carbonate mineral which is suitable for transportation; in this method a calcium carbonate mineral is mixed with a dispersing agent and with sufficient water to give a solids content in the range which is required for the final suspension, generally from 70% to 85% by weight of solids, and is then ground with a particulate grinding medium for a time sufficient to produce the required particle size distribution in the calcium carbonate mineral. This method has the disadvantage that the solids content which is required for the final suspension is unlikely to be the solids content at which the grinding process is most efficient and a considerable amount of energy may therefore be wasted in the grinding operation.